System and method for producing copper powder by electrowinning using the ferrous/ferric anode reaction

ABSTRACT

The present invention relates, generally, to a method for electrowinning copper powder, and more particularly to a method for electrowinning copper powder from a copper-containing solution using the ferrous/ferric anode reaction. In accordance with various embodiments of the present invention, a process for producing copper powder by electrowinning employs alternative anode reaction technology, namely, the ferrous/ferric anode reaction, and enables the efficient and cost-effective production of copper powder at a total cell voltage of less than about 1.5 V and at current densities of greater than 50 A/ft 2 . A copper powder electrowinning process in accordance with the present invention also reduces or eliminates acid mist generation that is characteristic of electrowinning operations utilizing conventional electrowinning chemistry (e.g., oxygen evolution at the anode), which is advantageous.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/160,910, filed on Jul. 14, 2005 and entitled, “System And Method ForProducing Copper Powder By Electrowinning Using The Ferrous/Ferric AnodeReaction.” The '910 application is a continuation-in-part of U.S.application Ser. No. 10/629,497, now U.S. Pat. No. 7,378,011, filed onJul. 28, 2003 and entitled “Method & Apparatus for Electrowinning CopperUsing the Ferrous/Ferric Anode Reaction.” The '497 application alsoclaims priority to U.S. Provisional Application No. 60/590,882, filed onJul. 22, 2004. All the above referenced applications are herebyincorporated by reference in their entirety.

FIELD OF INVENTION

This invention relates to a system and method for producing copperpowder by electrowinning. In particular, this invention relates to asystem and method for producing a copper powder product using theferrous/ferric anode reaction.

BACKGROUND OF THE INVENTION

Efficiency and cost-effectiveness of copper electrowinning is and for along time has been important to the competitiveness of the domesticcopper industry. Past research and development efforts in this area havethus focused—at least in part—on mechanisms for decreasing the totalenergy requirement for copper electrowinning, which directly impacts thecost-effectiveness of the electrowinning process.

In conventional copper electrowinning processes the following reactionsoccur:

Cathode Reaction:

Cu²⁺+SO₄ ²⁻+2e ⁻→Cu⁰+SO₄ ²⁻ (E ⁰=+0.345 V)

Anode Reaction:

H₂O→½O₂+2H⁺+2e ⁻ (E ⁰=−1.230 V)

Overall Cell Reaction:

Cu²⁺+SO₄ ²⁻+H₂O→Cu⁰+2H⁺+SO₄ ²⁻+½O₂ (E ⁰=−0.885 V)

Copper electrowinning according to the above reactions, while desirablein some applications, exhibits several areas of potential improvementfor, among other things, improved economics, increased efficiency, andreduced acid mist generation. First, in conventional copperelectrowinning, the decomposition of water reaction at the anodeproduces oxygen (O₂) gas. When the liberated oxygen gas bubbles breakthe surface of the electrolyte bath, they create an acid mist. Reductionor elimination of acid mist is desirable. Second, the decomposition ofwater anode reaction used in conventional electrowinning contributessignificantly to the overall cell voltage via the anode reactionequilibrium potential and the overpotential. The decomposition of wateranode reaction exhibits a standard potential of 1.23 Volts (V), whichcontributes significantly to the total voltage required for conventionalcopper electrowinning. The typical overall cell voltage is approximately2.0 V. Decreasing the anode reaction equilibrium potential and/oroverpotential can reduce cell voltage, and thus conserve energy bydecreasing the total operating costs of the electrowinning operation.

One way that has been found to potentially reduce the energy requirementfor copper electrowinning is to use alternative anode reactionelectrowinning (i.e., oxidation of ferrous ion to ferric ion at theanode), which occurs by the following reactions:

Cathode Reaction:

Cu²⁺+SO₄ ²⁻+2e ⁻→Cu⁰+SO₄ ²⁻ (E ⁰=+0.345 V)

Anode Reaction:

2Fe²⁺→2Fe³⁺+2e ⁻ (E⁰=−0.770 V)

Overall Cell Reaction:

Cu²⁺+SO₄ ²⁻+2Fe²⁺→Cu⁰+2Fe³⁺+SO₄ ²⁻ (E ⁰=−0.425 V)

The ferric iron generated at the anode as a result of this overall cellreaction can be reduced back to ferrous iron (i.e., “regenerated”) usingsulfur dioxide, as follows:

Solution Reaction:

2Fe³⁺+SO₂+2H₂O→2Fe²⁺+4H⁺+SO₄ ²⁻

The use of the ferrous/ferric anode reaction in copper electrowinningcells lowers the energy consumption of those cells as compared toconventional copper electrowinning cells that employ the decompositionof water anode reaction, since the oxidation of ferrous iron (Fe²⁺) toferric iron (Fe³⁺) occurs at a lower voltage than does the decompositionof water.

Conventional copper electrowinning processes produce solid coppercathode sheets. Copper powder, however, is an alternative to solidcopper cathode sheets. Production of copper powder as compared to coppercathode sheets can be advantageous in a number of ways. For example, itis potentially easier to remove and handle copper powder from anelectrowinning cell, as opposed to handling relatively heavy and bulkycopper cathode sheets.

In traditional electrowinning operations yielding copper cathode sheets,harvesting typically occurs every five to eight days, depending upon theoperating parameters of the electrowinning apparatus. Copper powderproduction has the potential, however, of being a continuous orsemi-continuous process, so harvesting may be performed on asubstantially continuous basis, therefore reducing the amount of“work-in-process” inventory as compared to conventional copper cathodeproduction facilities. Also, there is potential for operating copperelectrowinning processes at higher current densities when producingcopper powder than with conventional electrowinning processes thatproduce copper cathode sheets, capital costs for the electrowinning cellequipment may be less on a per unit of production basis, and it also maybe possible to lower operating costs with such processes. It is alsopossible to electrowin copper effectively from solutions containinglower concentrations of copper than using conventional electrowinning atacceptable efficiencies. Moreover, copper powder exhibits superiormelting characteristics over copper cathode sheets and copper powder maybe used in a wider variety of products and applications than canconventional copper cathode sheets. For example, it may be possible todirectly form rods, shapes, and other copper and copper alloy productsfrom copper powder. Copper powder can also be melted directly orbriquetted prior to melting and conventional rod production.

Although, in general, the use of the ferrous/ferric anode reaction inconnection with copper electrowinning is known, use of theferrous/ferric anode reaction in connection with the electrowinning ofcopper powder is not known. What is needed is an effective and efficientprocess for producing a high-quality, saleable copper powder productthat exhibits the advantages of alternative anode reactionelectrowinning chemistry.

SUMMARY OF THE INVENTION

In accordance with various embodiments of the present invention, copperpowder may be produced and harvested using alternative anode reactionelectrowinning chemistry (i.e., oxidation of ferrous ion to ferric ionat the anode), including using direct electrowinning techniques (i.e.,electrowinning copper from copper-containing solution without the use ofsolvent extraction or without the use of other methods for concentrationof copper in solution, such as ion exchange, ion selective membranetechnology, solution recirculation, evaporation, and other methods).Additionally, various aspects of the present invention enableenhancements in process ergonomics and process safety while achievingimproved process economics.

While the way in which the present invention addresses the deficienciesof the prior art and provides the above-mentioned and other advantageswill be discussed in greater detail below, in general, the presentinvention provides a method for producing copper powder byelectrowinning using an alternative anode reaction process.

In accordance with various exemplary embodiments of the presentinvention, the process and apparatus for electrowinning copper powderfrom a copper-containing solution using alternative anode reactiontechnology may be configured to optimize copper powder particle sizeand/or size distribution, to optimize cell operating voltage, cellcurrent density, and overall power requirements, to maximize the ease ofharvesting copper powder from the cathode, and/or to optimize copperconcentration in the lean electrolyte stream leaving the electrowinningoperation.

In accordance with other aspects of the invention, process stages andoperating parameters are designed to optimize copper powder quality,particularly with regard to the level of surface oxidation of the copperpowder particles, and, optionally, the particle size distribution andphysical properties of the final copper powder product.

The present invention relates to a copper electrowinning processdesigned to address, among other things, various deficiencies in priorart electrowinning systems and provide a method of producing copperpowder by electrowinning using the ferrous/ferric anode reaction. Theprocess disclosed herein achieves an advancement in the art by providinga copper powder electrowinning system that, by utilizing theferrous/ferric anode reaction in combination with other aspects of theinvention, enables significant enhancement in electrowinning efficiency,energy consumption, and reduction of acid mist generation as compared toother copper electrowinning processes. As used herein, the term“alternative anode reaction” refers to the ferrous/ferric anodereaction, and the term “alternative anode reaction process” refers toany electrowinning process in which the ferrous/ferric anode reaction isemployed.

These and other advantages of a process according to various aspects andembodiments of the present invention will be apparent to those skilledin the art upon reading and understanding the following detaileddescription with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The subject matter of the present invention is particularly pointed outand distinctly claimed in the concluding portion of the specification. Amore complete understanding of the present invention, however, may bestbe obtained by referring to the detailed description and claims whenconsidered in connection with the drawing figures, wherein like numeralsdenote like elements and wherein:

FIG. 1 is a flow diagram illustrating various aspects of a process forproducing copper powder in accordance with one exemplary embodiment ofthe present invention;

FIG. 2 is a flow diagram illustrating various aspects of a process forproducing copper powder in accordance with another exemplary embodimentof the present invention;

FIG. 3 is a flow diagram illustrating various aspects of anelectrowinning process in accordance with an exemplary embodiment of thepresent invention; and

FIGS. 4 through 9 illustrate results from experimentation with variouscathode configurations and process parameters in accordance with variousaspects of exemplary embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The detailed description of various exemplary embodiments of theinvention herein makes reference to the accompanying drawing figures.While these exemplary embodiments are described in sufficient detail toenable those skilled in the art to practice the invention, it should beunderstood that other embodiments may be realized and that logical andmechanical changes may be made without departing from the spirit andscope of the invention. Thus, the detailed description herein ispresented for purposes of illustration only and not of limitation. Forexample, the steps recited in any of the method or process descriptionsmay be executed in any order and are not limited to the order presented.

As an initial matter, it should be understood that various embodimentsof the present invention may be successfully employed to produce highquality copper powder from copper-containing solutions using alternativeanode reaction electrowinning chemistry.

In general, processes and systems configured according to variousembodiments of the present invention enable the efficient andcost-effective utilization of the alternative anode reaction inelectrowinning of copper powder at a cell voltage of less than about 1.5V. Furthermore, the use of such processes and/or systems reducesgeneration of acid mist.

With initial reference to FIG. 1, copper powder process 100 comprises anelectrowinning stage 1010 in which copper powder is electrowon from acopper-containing solution 101 using the ferrous/ferric anode reactionto produce a copper powder slurry stream 102.

In accordance with one aspect of an exemplary embodiment of theinvention, a copper-containing solution 101 is introduced into anelectrowinning cell, and copper is electrowon from the solution to formcopper powder. A copper powder slurry stream 102, which comprises thecopper powder product and electrolyte, is collected and removed from theelectrowinning cell, while a lean electrolyte stream 108 exits theelectrowinning cell from a side or top portion of the apparatus,preferably from an area generally opposite the entry point of thecopper-containing solution to the apparatus. Optionally, in accordancewith one exemplary embodiment of the invention, the lean electrolyteexiting the electrowinning apparatus may be subjected to filtration toremove suspended copper particles before being recycled to theelectrowinning apparatus, utilized in other processing areas, ordisposed of. Moreover, the rich electrolyte entering the electrowinningapparatus may be subjected to filtration prior to electrowinning toremove any undesirable solid and/or liquid impurities (including organicliquid impurities). When utilized, the degree of filtration desiredgenerally will be determined by the purity needs of the final copperpowder product (in the case of filtration prior to electrowinning), theneeds of other processing operations, and/or the amount of solid and/orliquid impurities present in the stream(s).

In accordance with one aspect of the invention, ferrous iron, forexample, in the form of ferrous sulfate (FeSO₄), is added to thecopper-rich electrolyte to be subjected to electrowinning, to cause theferrous/ferric (Fe²⁺+/Fe³⁺) couple to become the anode reaction. In sodoing, the ferrous/ferric anode reaction may replace the decompositionof water anode reaction (i.e., “conventional” electrowinning chemistry).As discussed above, because there is no oxygen gas produced in theferrous/ferric anode reaction, generation of “acid mist” as a result ofthe reactions in the electrowinning cell is eliminated. In addition,because the equilibrium potential of the Fe²⁺/Fe³⁺ couple (i.e.,E⁰=−0.770 V) is less than that for the decomposition of water (i.e.,E⁰=−1.230 V), the cell voltage is decreased, thereby decreasing cellenergy consumption.

Anode Characteristics

In general, any anode configuration now known or hereafter devisedsuitable to achieve the processing parameters and objectives describedherein may be used in accordance with various embodiments of the presentinvention. As such, various embodiments of the present invention mayutilize conventional “plate”-type (i.e., non-flow-through) anodes, otherflow-through or non-flow-through anodes of various geometries (e.g.,cylindrical anodes), flow-through anodes, or a combination of typeswithin one or more electrowinning cells. Any anode, however, thatenables electrowinning of copper powder from a copper-containingsolution using the ferrous/ferric anode reaction may be employed inconnection with the present invention.

In accordance with one exemplary embodiment of the invention, however,at least one flow-through anode is utilized in connection with theelectrowinning cell. As used herein, the term “flow-through anode”refers to any anode configured to enable electrolyte to pass through it.While fluid flow from the electrolyte entry point and/or any otherelectrolyte flow mechanisms incorporated into the electrowinning cellprovide electrolyte movement, a flow-through anode allows theelectrolyte in the electrowinning cell to flow through the anode duringthe electrowinning process.

When a flow-through anode is utilized, any now known or hereafterdevised flow-through anode may be utilized in accordance with variousexemplary embodiments of the present invention. Possible configurationsinclude, but are not limited to, metal, metal wool, metal fabric, othersuitable conductive nonmetallic materials (e.g., carbon materials), anexpanded porous metal structure, metal mesh, expanded metal mesh,corrugated metal mesh, multiple metal strips, multiple metal wires orrods, woven wire cloth, perforated metal sheets, and the like, orcombinations thereof. Moreover, suitable anode configurations are notlimited to planar configurations, but may include any suitablemultiplanar geometric configuration.

Anodes employed in conventional electrowinning operations typicallycomprise lead or a lead alloy, such as, for example, Pb—Sn—Ca. Onesignificant disadvantage of using such anodes is that, during theelectrowinning operation, small amounts of lead are released from thesurface of the anode and ultimately cause the generation of undesirablesediments, “sludges,” particulates suspended in the electrolyte, othercorrosion products, or other physical degradation products in theelectrowinning cell and contamination of the copper product. Forexample, copper produced in operations employing a lead-containing anodetypically comprises lead contaminant at a level of from about 0.5 ppm toabout 15 ppm. In accordance with one aspect of a preferred embodiment ofthe present invention, the anode is substantially lead-free. Thus,generation of lead-containing sediments, “sludges,” particulatessuspended in the electrolyte, or other corrosion or physical degradationproducts and resultant contamination of the copper powder with lead fromthe anode is avoided. In conventional electrowinning processes usingsuch lead anodes, another disadvantage is the need for cobalt to controlthe surface corrosion characteristics of the anode, to control theformation of lead oxide, and/or to prevent the deleterious effects ofmanganese in the system.

In accordance with one aspect of an exemplary embodiment of theinvention, the anode is formed of one of the so-called “valve” metals,including titanium (Ti), tantalum (Ta), zirconium (Zr), or niobium (Nb).The anode may also be formed of other metals, such as nickel (Ni),stainless steel (e.g., Type 316, Type 316L, Type 317, Type 310, etc.),or a metal alloy (e.g., a nickel-chrome alloy), intermetallic mixture,or a ceramic or cermet containing one or more valve metals. For example,titanium may be alloyed with nickel, cobalt (Co), iron (Fe), manganese(Mn), or copper (Cu) to form a suitable anode. Preferably, in accordancewith one exemplary embodiment, the anode comprises titanium, because,among other things, titanium is rugged and corrosion-resistant. Titaniumanodes, for example, when used in accordance with various embodiments ofthe present invention, potentially have useful lives of up to fifteenyears or more.

The anode may also optionally comprise any electrochemically activecoating. Exemplary coatings include those provided from platinum,ruthenium, iridium, or other Group VIII metals, Group VIII metal oxides,or compounds comprising Group VIII metals, and oxides and compounds oftitanium, molybdenum, tantalum, and/or mixtures and combinationsthereof. Ruthenium oxide and iridium oxide are two preferred compoundsfor use as an electrochemically active coating on titanium anodes.

In accordance with another aspect of an exemplary embodiment of theinvention, the anode comprises a titanium mesh (or other metal, metalalloy, intermetallic mixture, or ceramic or cermet as set forth above)upon which a coating comprising carbon, graphite, a mixture of carbonand graphite, a precious metal oxide, or a spinel-type coating isapplied. Preferably, in accordance with one exemplary embodiment, theanode comprises a titanium mesh with a coating comprised of a mixture ofcarbon black powder and graphite powder.

In accordance with an exemplary embodiment of the invention, the anodecomprises a carbon composite or a metal-graphite sintered materialwherein the exemplary metal described is titanium. In accordance withother embodiments of the invention, the anode may be formed of a carboncomposite material, graphite rods, graphite-carbon coated metallic meshand the like. Moreover, a metal in the metallic mesh or metal-graphitesintered exemplary embodiment is described herein and shown by exampleusing titanium; however, any metal may be used without detracting fromthe scope of the present invention. Exemplary embodiments of such anodesare set forth in various of the Examples herein.

In accordance with one exemplary embodiment, a wire mesh may be weldedto the conductor rods, wherein the wire mesh and conductor rods maycomprise materials as described above for anodes. In one exemplaryembodiment, the wire mesh comprises of a woven wire screen with 80 by 80strands per square inch, however various mesh configurations may beused, such as, for example, 30 by 30 strands per square inch. Moreover,various regular and irregular geometric mesh configurations may be used.In accordance with yet another exemplary embodiment, a flow-throughanode may comprise a plurality of vertically-suspended stainless steelrods, or stainless steel rods fitted with graphite tubes or rings. Inaccordance with another aspect of an exemplary embodiment, the hangerbar to which the anode body is attached comprises copper or a suitablyconductive copper alloy, aluminum, or other suitable conductivematerial.

Cathode Characteristics

In general, any cathode configuration now known or hereafter devisedsuitable to achieve the processing parameters and objectives describedherein may be used in accordance with various embodiments of the presentinvention. As such, various embodiments of the present invention mayutilize conventional “plate”-type (i.e., non-flow-through) cathodes,flow-through cathodes, or a combination of types within one or moreelectrowinning cells. Any cathode, however, that enables electrowinningof copper powder from a copper-containing solution may be employed inconnection with the present invention.

In accordance with one exemplary embodiment of the invention, however,at least one flow-through cathode is utilized in connection with theelectrowinning cell. For purposes of this detailed description ofpreferred embodiments of the invention, the term “cathode” refers to acomplete positive electrode assembly (typically connected to a singlebar). For example, in a cathode assembly comprising multiple thin rodssuspended from a bar, the term “cathode” is used to refer to the groupof thin rods, and not to a single rod. In accordance with one exemplaryembodiment of the present invention, a flow-through cathode isincorporated into the electrowinning apparatus. As used herein, the term“flow-through cathode” refers to any cathode configured to enableelectrolyte to pass through it.

Various flow-through cathode configurations may be suitable, including:(1) multiple parallel metal wires, thin rods, including hexagonal rodsor other geometries, (2) multiple parallel metal strips either alignedwith electrolyte flow or inclined at an angle to flow direction, (3)metal mesh, (4) expanded porous metal structure, (5) metal wool orfabric, and/or (6) conductive polymers. The cathode may be formed ofcopper, copper alloy, stainless steel, titanium, aluminum, or any othermetal or combination of metals and/or other materials. The surfacefinish of the cathode (e.g., whether polished or unpolished) may affectthe harvestability of the copper powder. Accordingly, polishing or othersurface finishes, surface coatings, surface oxidation layer(s), or anyother suitable barrier layer may advantageously be employed to enhanceharvestability.

In accordance with various embodiments of the present invention, thecathode may be configured in any manner now known or hereafter devisedby the skilled artisan.

All or substantially all of the total surface area of the portion of thecathode that is immersed in the electrolyte during operation of theelectrowinning cell is referred to herein, and generally in theliterature, as the “active” surface area of the cathode. This is theportion of the cathode onto which copper powder is formed duringelectrowinning.

In accordance with one aspect of an exemplary embodiment of theinvention, an electrowinning apparatus comprises multiple electrowinningcells configured in series or otherwise electrically connected, eachcomprising a series of electrodes-alternating anodes and cathodes. Inaccordance with one aspect of an exemplary embodiment, eachelectrowinning cell or portion of an electrowinning cell comprisesbetween about 4 and about 80 anodes and between about 4 and about 80cathodes. In accordance with one aspect of another exemplary embodiment,each electrowinning cell or portion of an electrowinning cell comprisesfrom about 15 to about 40 anodes and about 16 to about 41 cathodes.However, it should be appreciated that in accordance with the presentinvention, any number of anodes and/or cathodes may be utilized.

Each electrowinning cell or portions of each electrowinning cell maypreferably be configured with a base portion having a collectingconfiguration, such as, for example, a conical-shaped or trench-shapedbase portion, which collects the copper powder product harvested fromthe cathodes for removal from the electrowinning cell.

In accordance with one aspect of an exemplary embodiment of theinvention, the anodes and cathodes in the electrowinning cell are spacedevenly across the cell, and are maintained as close as possible tooptimize power consumption and mass transfer while minimizing electricalshort-circuiting of current between the electrodes. While anode/cathodespacing in conventional electrowinning cells is typically about 2 inchesor greater from anode to cathode, electrowinning cells configured inaccordance with various aspects of the present invention preferablyexhibit anode/cathode spacing of from about 0.5 inch to about 4 inches,and preferably less than about 2 inches. More preferably, anelectrowinning cell configured in accordance with an exemplaryembodiment of the present invention exhibits anode/cathode spacing ofabout or less than about 1.5 inches. As used herein, “anode/cathodespacing” is measured from the centerline of an anode hanger bar to thecenterline of the adjacent cathode hanger bar.

Electrolyte Flow Characteristics

While various configurations and combinations of anodes and cathodes inthe electrowinning cell may be used effectively in connection withvarious embodiments of the invention, preferably a flow-through anode isused and electrolyte circulation is provided using an electrolyte flowmanifold capable of maintaining satisfactory flow and circulation ofelectrolyte within the electrowinning cell. Generally speaking, anyelectrolyte pumping, circulation, or agitation system capable ofmaintaining satisfactory flow and circulation of electrolyte between theelectrodes in an electrowinning cell such that the processspecifications described herein are practical may be used in accordancewith various embodiments of the invention.

In accordance with an exemplary embodiment of the invention, theelectrolyte flow rate is maintained at a level of from about 0.05gallons per minute per square foot of active cathode to about 30 gallonsper minute per square foot of active cathode. Preferably, theelectrolyte flow rate is maintained at a level of from about 0.1 gallonsper minute per square foot of active cathode to about 0.75 gallons perminute per square foot of active cathode. It should be recognized thatthe optimal operable electrolyte flow rate useful in accordance with thepresent invention will depend upon the specific configuration of theprocess apparatus employed, and thus flow rates in excess of about 30gallons per minute per square foot of active cathode or less than about0.05 gallons per minute per square foot of active cathode may be optimalin accordance with various embodiments of the present inventionMoreover, electrolyte movement within the cell may be augmented byagitation, such as through the use of mechanical agitation and/orgas/solution injection devices, to enhance mass transfer.

Cell Voltage

In accordance with an exemplary embodiment of the invention, overallcell voltage of from about 0.5 V to about 1.5 V is achieved, preferablyfrom about 0.6 V to about 1.1 V, and more preferably from about 0.7 V toabout 0.9 V. Through the use of alternate anode reaction chemistry,overall cell voltages that are generally less than those achievablethrough conventional electrowinning reaction chemistry may be utilized.The mechanism for optimizing cell voltage within the electrowinning cellwill vary in accordance with various exemplary aspects and embodimentsof the present invention.

The overall cell voltage achievable generally is dependent upon a numberof interrelated factors, including electrode spacing, the configurationand materials of construction of the electrodes, acid concentration andcopper concentration in the electrolyte, current density, electrolytetemperature, and, to a smaller extent, the nature and amount of anyadditives to the electrowinning process (such as, for example,flocculants, surfactants, and the like).

In addition, the present inventors have recognized that independentcontrol of anode and cathode current densities, together with managingvoltage overpotentials, can be utilized to enable effective control ofoverall cell voltage and current efficiency. For example, theconfiguration of the electrowinning cell hardware, including, but notlimited to, the ratio of cathode surface area to anode surface area, canbe modified in accordance with the present invention to optimize celloperating conditions, current efficiency, and overall cell efficiency.

Current Density

The operating current density of the electrowinning cell affects themorphology of the copper powder product and directly affects theproduction rate of copper powder within the cell. In general, highercurrent density decreases the bulk density and particle size of thecopper powder and increases surface area of the copper powder, whilelower current density increases the bulk density of copper product(sometimes resulting in cathode copper if too low, which generally isundesirable in a copper powder electrowinning operation). For example,the production rate of copper powder by an electrowinning cell isapproximately proportional to the current applied to that cell—a celloperating at, say, 100 A/ft² of active cathode produces approximatelyfive times as much copper powder as a cell operating at 20 A/ft² ofactive cathode, all other operating conditions, including active cathodearea, remaining constant. The current-carrying capacity of the cellfurniture is, however, one limiting factor. Also, when operating anelectrowinning cell at a high current density, the electrolyte flow ratethrough the cell may need to be adjusted so as not to deplete theavailable copper in the electrolyte for electrowinning. Moreover, a celloperating at a high current density may have a higher power demand thana cell operating at a low current density, and as such, economics alsoplays a role in the choice of operating parameters and optimization of aparticular process.

In accordance with an exemplary embodiment of the invention, a currentdensity of from about 10 to about 700 amps per square foot of activecathode is maintained, preferably from about 50 A/ft² to about 150 A/ft²of active cathode. It should be recognized, however, that the maximumoperable current density achievable in accordance with variousembodiments of the present invention will depend upon the specificconfiguration of the process apparatus, the electrolyte flow rate, andother process parameters, and thus an operating current density inexcess of 700 A/ft² of active cathode may be achievable in accordancewith the present invention.

Temperature

In accordance with one aspect of an exemplary embodiment of the presentinvention, the temperature of the electrolyte in the electrowinning cellis maintained at from about 40° F. to about 150° F. In accordance withone preferred embodiment, the electrolyte is maintained at a temperatureof from about 90° F. to about 140° F. Higher temperatures may, however,be advantageously employed. For example, in direct electrowinningoperations, temperatures higher than 140° F. may be utilized.Alternatively, in certain applications, lower temperatures mayadvantageously be employed. For example, when direct electrowinning ofdilute copper-containing solutions is desired, temperatures below 85° F.may be utilized.

The operating temperature of the electrolyte in the electrowinning cellmay be controlled through any one or more of a variety of means wellknown in the art, including, for example, heat exchange, an immersionheating element, an in-line heating device (e.g., a heat exchanger), orthe like, preferably coupled with one or more feedback temperaturecontrol means for efficient process control.

Acid Concentration

In accordance with an exemplary embodiment of the present invention, theacid concentration in the electrolyte for electrowinning may bemaintained at a level of from about 5 to about 200 grams of acid perliter of electrolyte. In accordance with one aspect of a preferredembodiment of the present invention, the acid concentration in theelectrolyte is advantageously maintained at a level of from about 100 toabout 180 grams of acid per liter of electrolyte, depending upon theupstream process. Optimally, the acid concentration in the electrolyteis advantageously maintained at a level of from about 150 to about 160grams of acid per liter of electrolyte.

Copper Concentration

In accordance with an exemplary embodiment of the present invention, thecopper concentration in the electrolyte for electrowinning isadvantageously maintained at a level of from about 5 to about 40 gramsof copper per liter of electrolyte. Preferably, the copper concentrationis maintained at a level of from about 10 g/L to about 30 g/L.Optimally, the copper concentration in the electrolyte is advantageouslymaintained at a level of from about 15 to about 25 grams per liter ofelectrolyte. However, various aspects of the present invention may bebeneficially applied to processes employing copper concentrations aboveand/or below these levels, with lower copper concentration levels offrom about 0.5 to about 5 g/L and upper copper concentration levels offrom about 40 g/L to about 50 g/L being applied in some cases.

Iron Concentration

In accordance with an exemplary embodiment of the present invention, thetotal iron concentration in the electrolyte is maintained at a level offrom about 10 to about 60 grams of iron per liter of electrolyte.Preferably, the total iron concentration in the electrolyte ismaintained at a level of from about 30 g/L to about 50 g/L, and morepreferably, from about 40 g/L to about 50 g/L. It is noted, however,that the total iron concentration in the electrolyte may vary inaccordance with various embodiments of the invention, as total ironconcentration is a function of iron solubility in the electrolyte. Ironsolubility in the electrolyte varies with other process parameters, suchas, for example, acid concentration, copper concentration, andtemperature. As explained hereinabove, decreasing iron concentration inthe electrolyte is generally economically desirable, because doing sodecreases iron make-up requirements and decreases the electrolytesulfate saturation temperature, and thus decreases the cost of operatingthe electrowinning cell.

In accordance with an exemplary embodiment, wherein directelectrowinning is utilized in connection with the present invention, thetotal iron concentration in the electrolyte is maintained at a suitablelevel, preferably as much ferrous iron as can be retained, typically onthe order of from about 0.001 to about 10 grams of iron per liter ofelectrolyte.

As ferrous iron is oxidized at the anode in the electrowinning cell, theconcentration of ferrous iron in the electrolyte is naturally depleted,while the concentration of ferric iron in the electrolyte is naturallyincreased. In accordance with one aspect of an exemplary embodiment ofthe invention, the concentration of ferrous iron in the electrolyte iscontrolled by addition of ferrous sulfate to the electrolyte. Inaccordance with another embodiment of the invention, the concentrationof ferrous iron in the electrolyte is controlled by solution extraction(SX) of iron from copper leaching solutions. Ferrous/ferric ions alsocan be leached from the ore, concentrate, or any other iron source(e.g., pyrite, marcasite, pyrrhotite, scrap iron) to generate additionaliron, preferably in the form of ferrous ions that do not requireregeneration prior to addition to the electrolyte.

In order for the ferrous/ferric couple to maintain a continuous anodereaction, the ferric iron generated at the anode preferably is reducedback to ferrous iron to maintain a satisfactory ferrous concentration inthe electrolyte. Additionally, the ferric iron concentration preferablyis controlled to achieve satisfactory current efficiency in theelectrowinning cell.

In accordance with an exemplary embodiment, the ferric ironconcentration in the electrolyte is maintained at a level of from about0.001 to about 10 grams of iron per liter of electrolyte. Preferably,the ferric iron concentration in the electrolyte is maintained at alevel of from about 1 g/L to about 6 g/L, and more preferably, fromabout 2 g/L to about 4 g/L.

With reference to FIG. 3, in accordance with another aspect of anexemplary embodiment of the invention, the concentration of ferric ironin the electrolyte within the electrowinning cell is controlled byremoving at least a portion of the electrolyte from the electrowinningcell, for example, as illustrated in FIG. 3 as electrolyte regenerationstream 35 of process 300.

In accordance with one aspect of an exemplary embodiment of theinvention, sulfur dioxide 37 may be used to reduce the ferric iron inelectrolyte regeneration stream 35. Although reduction of Fe³⁺ to Fe²⁺in electrolyte regeneration stream 35 in ferrous regeneration stage 303may be accomplished using any suitable reducing reagent or method,sulfur dioxide is particularly attractive as a reducing agent for Fe³⁺because it is generally available from other copper processingoperations, and because sulfuric acid is generated as a byproduct. Uponreacting with ferric iron in a copper-containing electrolyte, the sulfurdioxide is oxidized, forming sulfuric acid. The reaction of sulfurdioxide with ferric iron produces two moles of sulfuric acid for eachmole of copper produced in the electrowinning cell, which is one molemore of acid than is typically required to maintain the acid balancewithin the overall copper extraction process, when solution extraction(SX) is used in conjunction with electrowinning. The excess sulfuricacid may be extracted from the acid-rich electrolyte (illustrated inFIG. 3 as stream 38) generated in the ferrous regeneration stage for usein other operations, such as, for example, leaching operations.

With further reference to FIG. 3, the acid-rich electrolyte stream 38from ferrous regeneration stage 301 may be returned to electrowinningstage 1010 via electrolyte recycle streams 32 and 36, may be introducedto acid removal stage 302 for further processing, or may be split (asshown in FIG. 3) such that a portion of acid-rich electrolyte stream 38returns to electrowinning stage 1010 and a portion continues to acidremoval stage 302. In acid removal stage 302, excess sulfuric acid isextracted from the acid-rich electrolyte and leaves the process via acidstream 39, to be neutralized or, preferably, used in other operations,such as, for example a heap leach operation. The acid-reducedelectrolyte stream 34 may then be returned to electrowinning stage 1010via electrolyte recycle stream 36, as shown in FIG. 3.

In accordance with another aspect of an exemplary embodiment of theinvention, the ferric-rich electrolyte is contacted with sulfur dioxidein the presence of a catalyst, such as, for example, activated carbonmanufactured from bituminous coal, or other types of carbon with asuitable active surface and suitable structure. The reaction of sulfurdioxide and ferric iron is preferably monitored such that theconcentration of ferric iron and ferrous iron in the acid-richelectrolyte stream produced in the ferrous regeneration stage can becontrolled. In accordance with an aspect of another embodiment of theinvention, two or more oxidation-reduction potential (ORP) sensors areused—at least one ORP sensor in the ferric-rich electrolyte lineupstream from the injection point of sulfur dioxide, and at least oneORP sensor downstream from the catalytic reaction point in theferric-lean electrolyte. The ORP measurements provide an indication ofthe ferric/ferrous ratio in the solution; however, the exactmeasurements depend on overall solution conditions that may be unique toany particular application. Those skilled in the art will recognize thatany number of methods and/or apparatus may be utilized to monitor andcontrol the ferric/ferrous ratio in the solution. The ferric-richelectrolyte will contain from about 0.001 to about 10 grams per literferric iron, and the ferric-lean electrolyte will contain up to about 6grams per liter ferric iron.

Harvest of Copper Powder

While in situ harvesting configurations may be desirable to minimizemovement of cathodes and to facilitate the removal of copper powder on acontinuous or semi-continuous basis, any number of mechanisms may beutilized to harvest the copper powder product from the cathode inaccordance with various aspects of the present invention. Any device nowknown or hereafter devised that functions to facilitate the release ofcopper powder from the surfaces of the cathode to the base portion ofthe electrowinning apparatus, enabling collection and further processingof the copper powder in accordance with other aspects of the presentinvention, may be used. The optimal harvesting mechanism for aparticular embodiment of the present invention will depend largely on anumber of interrelated factors, primarily current density, copperconcentration in the electrolyte, electrolyte flow rate, and electrolytetemperature. Other contributing factors include the level of mixingwithin the electrowinning apparatus, the frequency and duration of theharvesting method, and the presence and amount of any process additives(such as, for example, flocculant, surfactants, and the like).

In situ harvesting configurations, either by self-harvesting (describedbelow) or by other in situ devices, may be desirable to minimize theneed to remove and handle cathodes to facilitate the removal of copperpowder from the electrowinning cell. Moreover, in situ harvestingconfigurations may advantageously permit the use of fixed electrode celldesigns. As such, any number of mechanisms and configurations may beutilized.

Examples of possible harvesting mechanisms include vibration (e.g., oneor more vibration and/or impact devices affixed to one or more cathodesto displace copper powder from the cathode surface at predetermined timeintervals), a pulse flow system (e.g., electrolyte flow rate increaseddramatically for a short time to displace copper powder from the cathodesurface), use of a pulsed power supply to the cell, use of ultrasonicwaves, and use of other mechanical displacement means to remove copperpowder from the cathode surface, such as intermittent or continuous airbubbles. Alternatively, under some conditions, “self-harvest” or“dynamic harvest” may be achievable, when the electrolyte flow rate issufficient to displace copper powder from the cathode surface as it isformed, or shortly after deposition and crystal growth occurs.

In accordance with an aspect of one embodiment of the invention, finecopper powder that is carried through the cell with the electrolyte isremoved via a suitable filtration, sedimentation, or other finesremoval/recovery system.

In general, according to various aspects of the present invention, aprocess for producing copper powder includes the steps of: (i)electrowinning copper powder from a copper-containing solution toproduce a slurry stream containing copper powder particles andelectrolyte; (ii) optionally, separating at least a portion of theelectrolyte from the copper powder particles in the slurry stream; (iii)conditioning the slurry stream; (iv) optionally, separating the bulk ofthe liquid from the copper powder particles; and (v) optionally, dryingthe copper powder particles originally present in the slurry stream toproduce a final, stable copper powder product.

Referring again to FIG. 1, in accordance with one aspect of an exemplaryembodiment of the invention, copper powder slurry stream 102 fromelectrowinning stage 1010 optionally is subjected to solid/liquidseparation (step 1020) to reduce the amount of electrolyte in stream102. Optional solid/liquid separation stage 1020 may comprise anyapparatus now known or hereafter developed for separating at least aportion of the electrolyte (stream 104) from the copper powder in copperpowder slurry stream 102, such as, for example, a clarifier, a spiralclassifier, other screw-type devices, a countercurrent decantation (CCD)circuit, a thickener, a filter, a conveyor-type device, a gravityseparation device, or other suitable apparatus. In accordance with oneaspect of an exemplary embodiment of the invention, the solid/liquidseparation apparatus chosen will enable separation of electrolyte fromthe copper powder while preventing exposure of the copper powder to air,which can cause rapid surface oxidation of the copper powder particles.

In accordance with an optional aspect of an exemplary embodiment of theinvention, at least a portion of electrolyte stream 104 leavingsolid/liquid separation stage 1020 may be recycled to the electrowinningcell (stream 112) and/or may be combined with lean electrolyte stream108 (stream 111).

In accordance with one embodiment of the invention, copper powder slurrystream 102 from electrowinning stage 1010 has a solids content of fromabout 5 percent by weight to about 30 percent by weight. However, thesolids content of copper powder slurry stream 102 from electrowinningstage 1010 is largely dependent upon the copper powder harvesting methodchosen in electrowinning stage 1010. Preferably, solid/liquid separationstage 1020, when used, is configured to produce a concentrated copperpowder slurry stream 103 that has a solids content of at least about 20percent by weight, and preferably greater than about 30 percent byweight, for example, in the range of about 60 percent to about 80percent by weight or more depending upon the bulk density and morphologyof the copper powder. High solids content may be advantageous,particularly if coarse or granular copper powder is harvested. It isgenerally desirable to separate as much electrolyte as possible from thecopper powder prior to subjecting the copper powder slurry stream tofurther processing, as doing so potentially reduces the cost ofdownstream processing (e.g., by reducing process stream volume and thuscapital and operating expenses) and potentially increases the quality ofthe final copper powder product (e.g., by reducing surface oxidation ofthe copper powder particles by the electrolyte and by reducing levels ofentrained impurities).

With continued reference to FIG. 1, in accordance with an exemplaryembodiment of the invention, after leaving solid/liquid separation stage1020, concentrated copper powder slurry stream 103 is subjected to aconditioning stage 1030 to further condition the copper powder inpreparation for drying. In accordance with various aspects of anexemplary embodiment, conditioning stage 1030, comprising one or moreprocessing steps, is configured to (i) adjust of the pH of concentratedcopper powder slurry stream 103, (ii) stabilize the surface of thecopper powder particles to prevent surface oxidation, and/or (iii)further reduce the amount of excess liquid in the copper powder slurrystream to form a moist copper powder product. Adjustment of the pH ofconcentrated copper powder slurry stream 103 and stabilization of thesurface of the copper powder particles in copper powder slurry stream103 is facilitated by the addition of one or more conditioning agents105 to conditioning stage 1030.

In accordance with one exemplary aspect of an embodiment of the presentinvention, conditioning stage 1030 comprises any apparatus now known orhereafter developed capable of achieving the above objectives, and, inparticular, capable of treating substantially all surfaces of the copperparticles reasonably equally with conditioning agents 105. In accordancewith one exemplary embodiment of the invention, conditioning stage 1030comprises use of a centrifuge. Exemplary processing parameters forconditioning stage 1030 are discussed hereinbelow in connection withanother embodiment of the present invention.

In accordance with one aspect of an exemplary embodiment of the presentinvention, it may be advantageous that a dewatering stage 1040 beemployed to enable a bulk of the liquid in copper powder stream 106 tobe separated from the bulk of the copper powder as economically aspossible. For example, a centrifuge, a filter, or other suitablesolid/liquid separation apparatus may be used. In accordance with oneaspect of this embodiment of the invention, this separation may beaccomplished during and/or in connection with conditioning the copperpowder slurry in conditioning stage 1030, such as in connection withconditioning stage 1030 when use of a centrifugal conditioning step iscarried out. Alternatively, in certain embodiments, additionaldewatering may be desired to yield a copper powder product that isuseable for future processing without additional conditioning and/orprocessing (e.g., drying).

With further reference to FIG. 1, after leaving optional dewateringstage 1040, copper powder stream 107 may be subjected to an optionaldrying stage 1050 to produce a final copper powder product stream 110.In accordance with an exemplary aspect of an embodiment of the presentinvention, drying stage 1050 comprises any apparatus now known orhereafter developed capable of drying the copper powder sufficiently forpackaging as a final product and/or for transfer to downstream processand for downstream processing steps for formation of alternative copperproducts. For example, drying stage 1050 may comprise a flash dryer, acyclone, a dry sintering apparatus, a conveyor belt dryer, and/or othersuitable apparatus. Furthermore, in cases where the copper powder is tobe melted (e.g., rod mill, shaft furnace, etc.), then the excess heatfrom the melting process may be used beneficially to dry the copperpowder product.

In accordance with another exemplary embodiment of the invention, aprocess for producing copper powder includes the steps of (i)electrowinning copper powder from a copper-containing solution toproduce a slurry stream containing copper powder particles andelectrolyte; (ii) optionally, separating at least a portion of theelectrolyte from the copper powder particles in the slurry stream; (iii)optionally, separating one or more coarse copper powder particle sizedistributions in the slurry stream from one or more finer copper powderparticle size distributions in the slurry stream in one or more sizeclassification stages; (iv) optionally, conditioning the slurry stream;(v) separating the bulk of the liquid from the copper powder particles;(vi) optionally, drying the copper powder particles in the slurry streamto produce a dry copper powder stream; (vii) optionally, separating oneor more coarse copper powder particle size distributions in the drycopper powder stream from one or more finer copper powder particle sizedistributions in the dry copper powder stream in one or more sizeclassification stages; and (viii) either collecting the copper powderfinal product from the process or subjecting the copper powder stream tofurther processing (such as, for example, briquetting, extrusion,melting, or other downstream processes).

Turning now to FIG. 2, copper powder process 200 exemplifies variousaspects of another embodiment of the present invention. In accordancewith the illustrated embodiment, a copper-containing solution 201 isprovided to an electrowinning stage 2010. Electrowinning stage 2010 isconfigured to produce a copper powder slurry stream 203, which comprisescopper powder and an electrolyte, and a lean electrolyte stream 202.Lean electrolyte stream 202 may be recycled to upstream processingoperations (such as, for example, an upstream leaching operation used toproduce copper-containing solution 201), used in other processingoperations, or impounded or disposed of. In cases where the copperproduct is to be melted, for example, in a rod meld shaft furnace, thenthe excess heat from the melting process may be used beneficially to drythe said copper product.

In accordance with one aspect of an exemplary embodiment of theinvention, copper powder slurry stream 203 then optionally undergoessolid/liquid separation in solid/liquid separation (or “dewatering”)stage 2020, which may, as described above in connection with FIG. 1,comprise any apparatus now known or hereafter developed for separatingat least a portion of the bulk electrolyte (stream 204) from the copperpowder in copper powder slurry stream 203, such as, for example, aclarifier, a spiral classifier, a screw-type device, a countercurrentdecantation (CCD) circuit, a thickener, a filter, a gravitationalseparator device, a conveyor-type device, or other suitable apparatus.Such an advantageous bulk liquid removal step may yield a copper powderproduct that is useable for future processing without additionalconditioning and/or processing. Preferably, semi-continuous copperpowder harvesting within the electrowinning cell is advantageouslymatched with batch downstream processing (i.e., dewatering andconditioning) such that copper powder product is more continuouslyrecovered. For example, multiple solid/liquid separation devices may beemployed in connection with a conditioning stage, and as such,downstream solid/liquid separation may be eliminated.

With further reference to FIG. 2, in accordance with an optional aspectof an embodiment of the present invention, the resulting concentratedcopper powder slurry from solid/liquid separation stage 2020 (stream205) may be collected in a copper powder slurry tank 2030. Copper powderslurry tank 2030 is configured to hold the concentrated copper slurryand to maintain homogeneity of the slurry through mixing, agitation, orother means. Additionally, process water 215 and/or a pH-adjusting agent216 (such as, for example, ammonium hydroxide) may optionally be addedto copper powder slurry tank to aid in maintaining homogeneity of theslurry, stabilizing the copper powder in the slurry, and/or adjustingthe pH of the slurry in preparation for further processing. Inaccordance with another aspect of an exemplary embodiment of theinvention, slurry tank 2030 is configured such that the copper powderslurry is not exposed to air during storage and/or treatment, as suchexposure may, as described above, detrimentally affect the surfaceintegrity of the copper powder particles.

Upon discharge from slurry tank 2030, slurry stream 206 may, optionally,undergo a size classification stage 2040. If utilized, the objective ofsize classification stage 2040 is to separate coarser copper powderparticles from finer copper powder particles in the slurry stream, inaccordance with specifications for the desired final copper powderproduct. For example, if the final copper powder product is to be usedfor extruding copper shapes or other products, such as by direct rotaryextrusion, a slurry stream comprising finer copper powder particles ispreferred, whereas if the final copper powder product is to be meltedfor rod or other product formation, relatively coarse copper powderparticles may be preferable. As used herein, the term “coarse” describescopper powder particles larger than about 200 microns (in the range ofabout plus 100 mesh). The term “fine” is used herein to describe copperpowder particles smaller than about 50 microns (in the range of aboutminus 325 mesh). Particles between those ranges are referred to as“intermediate” particles.

When size classification is desired, it may be carried out at anysuitable stage in the copper powder production process, the suitabilityof any stage being dependent upon a variety of factors, including thesize of the copper powder particles leaving the electrowinning stage,the configuration and materials of construction of the sizeclassification apparatus, and other engineering and economic processconsiderations.

In accordance with an exemplary embodiment of the invention, whenutilized, size classification may be conducted on the slurry streamleaving the electrowinning cell, the optional slurry tank (prior toconditioning), and/or on the copper powder product stream. Suchprocessing may allow for stabilization of fine particles and differenttreatment of coarser particles. In the event size classification isconducted, the different particle size distributions, or, if desired,various mixtures thereof, may be processed further, as will now bediscussed.

Referring again to FIG. 2, in accordance with an exemplary embodiment ofthe invention, after leaving the optional size classification stage2040, slurry stream 207 (or slurry stream 206, if size classification isnot utilized) is subjected to an optional conditioning operation 2050 tocondition the copper powder and/or the solution in preparation fordewatering and optional drying. In accordance with one exemplary aspectof an embodiment of the present invention, conditioning operation 2050,when used, may be performed in conjunction with a dewatering operation2060.

In accordance with one embodiment of the present invention, optionalconditioning operation 2050 may include washing, pH adjustment, removalof impurities, stabilization, and/or other conditioning operations.

In accordance with an exemplary embodiment of the invention, the copperslurry may be contacted with a washing agent 208 and/or a stabilizingagent 209. Washing agent 208 can comprise any liquid material, water,ammonium hydroxide, and/or mixtures thereof. Optionally, washing agent208 may include additional materials, such as, for example, surfactants,soaps, and the like. In accordance with one aspect of an exemplaryembodiment of the invention, washing agent 208 may be heated prior towashing, which may enhance impurity removal. Stabilizing agent 209 maybe any agent suitable for preventing surface oxidation of the copperpowder particles (which oxidation may diminish the value and/or qualityof the copper powder product and/or may negatively impact downstreamoperations or applications).

In accordance with various aspects of an exemplary embodiment,stabilizing agent 209 comprises an organic surfactant in combinationwith a stabilizer. The organic surfactant may be used to lower thesurface tension of the stabilizer and thus enable the stabilizer topenetrate into all pores of the copper powder particles. The stabilizer,on the other hand, preferably is the “active” agent that coats theparticles and prevents oxidation, thus providing a suitable shelf lifeto the copper powder product and enabling transfer of the copper powderin an otherwise oxidizing atmosphere (i.e., air). Some suitablestabilizers include, for example, 1,2,3-Benzotriazole (BTA), animalglue, fish glue, soaps, and the like. Under certain circumstances,however, the use of a stabilization agent may be unnecessary, such aswhen the copper powder product is intended to be processed immediatelyafter production (by melting and casting, for example). Moreover, othermethods of preventing surface oxidation of the copper powder particlesduring processing may reduce or eliminate the need for a stabilizationagent, such as, for example, use of a charged fluidized bed or use ofnitrogen blanketing during one or more stages of copper powder handling.If it is desirable to store the copper powder product for an extendedperiod of time, however, then a stabilizing agent may be desired.

In accordance with an exemplary aspect of an embodiment of the presentinvention, it is advantageous that a dewatering stage 2060 be employedto enable a bulk of the liquid in copper powder stream 211 to beseparated from the bulk of the copper powder as economically aspossible. For example, a centrifuge, a filter, or other suitablesolid/liquid separation apparatus may be used.

In accordance with one aspect of this embodiment of the invention, thisseparation may be accomplished during or in connection with conditioningthe copper powder slurry, such as in connection with optionalconditioning operation 2050. Such an advantageous dewatering step mayyield a copper powder product that is useable for future processingwithout additional conditioning and/or processing (e.g., drying). Inaccordance with an exemplary embodiment, after the copper powder iswashed and stabilized, a dewatering stage 2060 is utilized to draw asmuch liquid from copper powder slurry 211 as possible, producing a moistcopper powder stream 212. Moist copper powder stream 212 may then besubjected to an optional drying stage 2070 to produce a final copperpowder product stream 213.

In accordance with an exemplary aspect of an embodiment of the presentinvention, optional drying stage 2070 comprises any apparatus now knownor hereafter developed capable of drying the copper powder sufficientlyfor packaging as a final product and/or for shipping to downstreamprocess and for downstream processing steps for formation of alternativecopper products. For example, drying stage 2070 may comprise a flashdryer, a fluid bed dryer, a rotary dryer, a cyclone, a dry sinteringapparatus, a conveyor belt dryer, and/or other suitable apparatus fordirect or indirect drying. In accordance with an exemplary embodiment,optional drying stage 2070 comprises a flash dryer that enables rapiddrying of the copper powder particles without disturbing the integrityof the stabilizer coating on the copper powder particles. In dryingstage 2070, moist copper powder stream 212 is contacted with sufficienthot air for a period of time sufficient to reduce the moisture contentof the copper powder particles. The final moisture content of the copperpowder product stream 213 may vary, depending upon the nature of anydownstream processing of the copper powder (through, for example, sizeclassification, packaging, direct forming of copper shapes and rods,casting, briquetting, and the like). In this regard, in certainapplications, significant moisture content may be retained withoutdeleteriously impacting subsequent processing.

As mentioned above, and with further reference to FIG. 2, after leavingoptional drying stage 2070, copper powder product stream 213 mayoptionally undergo size classification in size classification stage 2080to achieve a desired particle size distribution in the final copperpowder product 214. The final copper powder product 214 may then be sentto a packaging operation 2090—for example, a bagging operation—or may besubjected to further processing 2095 to change the character of thefinal copper product.

EXAMPLES

A number of experiments were conducted in accordance with variousaspects of exemplary embodiments of the present invention usingalternative anode reaction chemistries. In particular, experimentationsought to evaluate harvestability conditions of copper powder in anelectrowinning cell. Cathode configurations used were of a flow-throughdesign and incorporated stainless steel and titanium rods with varyingdiameters, cross-sectional geometries, and surface finishes. The cathodeimmersed plating depth was 29 inches. Anodes used were constructed ofexpanded titanium mesh with an iridium oxide-based coating. Variouselectrolyte flow tubes were attached to the electrowinning cell to testdifferent electrolyte injection geometries. In the experimentsconducted, electrolyte flow was evenly distributed across the front ofthe cell. The electrolyte therefore flowed through the electrodes, notfrom side to side. A recirculation tank with an immersion heater andpump were attached to the electrowinning cell to provide electrolyte tothe cell. Process variables studied included current density,electrolyte flow rate, copper concentration, and iron concentration.

As described below and as illustrated in FIGS. 4 through 9, theconditions that most favored copper powder formation on the cathodeincluded low copper concentration, high current density, small diametercathode rods, the use of titanium cathodes, and low electrolyte flowrate. Conditions were found where copper powder that is easilyharvestable can be produced.

Table 1 contains the experimental design. Fixed variables for thisdesign included electrolyte chemistry (44 to 48 g/L copper, 22 to 26 g/Liron, 150 to 160 g/L sulfuric acid), and electrolyte temperature of 120°F. Dependent variables (measured variables) were cell voltage and theharvestability of the copper powder produced (i.e., whether theelectrowon copper was easily removed as powder, or whether it formedpseudo-cathode, a cathode-powder intermediate that tends to stick to thecathode). As the experimental results were obtained, new tests weredesigned to explore specific areas of interest. As a result of theelectrolyte chemistry and the anodes utilized in this testing, the anodereaction in the cell was the oxidation of ferrous to ferric iron.

TABLE 1 Exploratory experiment design for copper powder production.Current Electrolyte Flow Cathode Material Cathode Rod Density, A/ft²Rate, Gpm of Construction Diameter, inches 300 10 316 Stainless ⅛ Steel600 50 Titanium ¾

The harvesting method utilized in each test was to open 1-inch side flowports on the side of the electrowinning cell one by one for a shortperiod of time. This imposed an electrolyte flow of approximately 20 gpmon individual sections of the cathode rods, causing the electrowoncopper powder to fall off the cathode rods. If residual powder remainedon the cathode rods at the end of the harvest cycle, the entire cathodewas lifted and the powder was removed by hand. Harvest cycles weretypically 1 to 2 hours.

FIG. 4 illustrates the effect of current density on cell voltage andcopper powder harvestability using ¾-inch stainless steel or ¾-inchtitanium cathode rods, an electrolyte flow rate of 10 or 50 gpm, and anelectrolyte containing 44 to 48 g/L copper, 22 to 26 g/L iron, 150 to160 g/L sulfuric acid, and maintained at a temperature of 120° F.Additional current density points were added to the experiment designbecause of the pseudo-cathode plate that was evident at an operatingcurrent density of 300 A/ft². The transition from pseudo-cathode to anall-powder copper deposit occurred between about 400 A/ft² and about 500A/ft² at both 10 and 50 gpm electrolyte flow using stainless steelcathode rods. Cell voltage increased to above 2.5 volts at 500 A/ft².Oxygen evolution was evident at the anode at 600 and 700 A/ft²,indicating insufficient ferrous iron concentration in the electrolyte tomaintain the ferrous/ferric anode reaction at these current densities.

Titanium cathode rods were tested at 500, 600, and 700 A/ft² forcomparison. At 10 gpm electrolyte flow, the titanium rod cathodeexhibited an all-powder deposit and the electrowon copper powder wasmore easily harvested than when the ¾-inch stainless steel rod cathodewas tested under the same conditions. The test results with the ¾-inchtitanium cathode rods at 50 gpm are set forth in FIG. 5. When flow wasincreased from 10 to 50 gpm using the ¾-inch titanium rod cathode,pseudo-cathode plate was evident at 500 and 600 A/ft², whereas at 10 gpmelectrolyte flow, the electrowon copper was an all-powder deposit. Acurrent density of 700 A/ft² was required at the higher flow rate toproduce an all-powder deposit.

FIG. 6 sets forth the results obtained using smaller diameter cathoderods. In these experimental tests, ⅛-inch diameter titanium rods werenot immediately available, and so ¼-inch rods were substituted. A ¼-inchhexagonal titanium rod cathode was also tested. The tests were conductedat 10 gpm electrolyte flow only. As the results in FIG. 6 demonstrate,when using a cathode with smaller diameter rods, a lower current densitycan be operated while still maintaining an all-powder copper deposit.Pseudo-cathode plate was observed on the ⅛-inch stainless steel cathoderods at an operating current density of 100 A/ft², but not in the othertests displayed in FIG. 6.

Additional tests with ¼-inch titanium rod cathodes were completed toobserve whether changes in electrolyte flow rate could be used to causecopper powder to fall off the cathode rods over time as the powderparticles grew in size. To facilitate this, the copper concentration inthe electrolyte was decreased to 26 g/L. Iron concentration in theelectrolyte was increased to 40 g/L to help lower the cell voltage. Acurrent density of 150 A/ft² was utilized. The results are shown in FIG.7. As can be seen in FIG. 7, increasing electrolyte flow rate to 40 gpmcaused pseudo-cathode to form. At electrolyte flow rates of 10 and 20gpm, an all-powder deposit was produced, but the copper powder did notfall off cathode rods spontaneously. However, the copper powder wasremoved very easily from the cathode rods at the lower flow rates.

Smaller diameter titanium rod cathodes with polished surfaces (versusstandard milled surfaces) were then tested. In order to lower cellvoltage to 1 volt or less, a current density of 100 A/ft² and anelectrolyte iron concentration of 30 to 40 g/L were used. Copperconcentration was varied from 45 g/L to 15 g/L (with the ⅛-inch polishedtitanium cathode) to observe whether the copper powder would begin toharvest spontaneously at an electrolyte flow rate of 10 gpm. The resultsare set forth in FIG. 8. The copper produced was an all-powder depositand cell voltage was maintained at less than 1.0 V, but the powder didnot self-harvest. At a copper concentration of 45 g/L in theelectrolyte, only slight pseudo-cathode plating was observed on the⅛-inch polished titanium cathode.

FIG. 9 is a summary showing electrowinning power consumption as afunction of current density at 10 gpm electrolyte flow rate and 120° F.In order to operate at less than 0.5 W-hr/lb copper, a current densityof 100 A/ft² is indicated.

The present invention has been described above with reference to anumber of exemplary embodiments. It should be appreciated that theparticular embodiments shown and described herein are illustrative ofthe invention and its best mode and are not intended to limit in any waythe scope of the invention. Those skilled in the art having read thisdisclosure will recognize that changes and modifications may be made tothe exemplary embodiments without departing from the scope of thepresent invention. For example, various aspects and embodiments of thisinvention may be applied to electrowinning of metals other than copper,such as nickel, zinc, cobalt, and others. Although certain preferredaspects of the invention are described herein in terms of exemplaryembodiments, such aspects of the invention may be achieved through anynumber of suitable means now known or hereafter devised. Accordingly,these and other changes or modifications are intended to be includedwithin the scope of the present invention.

1. A system for producing copper powder by electrowinning comprising: anelectrolyte stream, wherein said electrolyte stream comprises copper andsolubilized ferrous iron; an electrowinning cell, wherein saidelectrowinning cell comprises at least one flow-through anode and atleast one flow-through cathode; wherein said electrowinning cell isconfigured to operate at a current density of at least 26 amperes persquare foot of active cathode; wherein said electrowinning cell isconfigured to oxidize at least a portion of said solubilized ferrousiron in said electrolyte stream from ferrous iron to ferric iron at theat least one flow-through anode.
 2. The system of claim 1, furthercomprising an electrolyte flow manifold for introducing said electrolytestream into said electrowinning cell.
 3. The system of claim 1, whereinsaid electrowinning cell is configured to operate at a cell voltage ofless than about 1.5 Volts.
 4. The system according to claim 1, whereinsaid electrolyte stream has a total iron concentration of from about 10g/L to about 60 g/L.
 5. The system according to claim 1, wherein saidelectrolyte stream has a copper concentration of from about 5 g/L toabout 40 g/L.
 6. The system according to claim 1, wherein saidelectrolyte stream has an acid concentration of from about 5 g/L toabout 200 g/L.
 7. The system according to claim 1, wherein saidelectrolyte stream has a temperature in the range of from about 40° F.to about 150° F.
 8. The system according to claim 1, further comprisinga regeneration vessel configured to reduce at least a portion of saidferric iron to ferrous iron to form a regenerated electrolyte stream. 9.The system according to claim 8, wherein said regeneration vesselcontains a catalyst.
 10. The system according to claim 8, wherein saidregeneration vessel further contains sulfur dioxide gas.
 11. The systemaccording to claim 1, wherein said electrolyte stream further comprisesferric iron in a concentration from about 0.001 g/L to about 10 g/L. 12.The system according to claim 1, wherein said at least one flow-throughanode comprises an anode comprising a metal mesh with anelectrochemically active coating.
 13. The system according to claim 1,wherein said at least one flow-through anode comprises an anodecomprising titanium mesh with an iridium-oxide based coating.
 14. Thesystem according to claim 1, wherein said at least one flow-throughanode comprises an anode comprising titanium mesh with a ruthenium-oxidebased coating.
 15. The system according to claim 2, wherein saidelectrolyte flow manifold is configured to maintain an electrolyte flowrate to said electrowinning cell of from about 0.05 gallons per minuteper square foot of active cathode to about 30 gallons per minute persquare foot of active cathode.
 16. The system according to claim 9,wherein said catalyst is activated carbon.
 17. The system according toclaim 1, wherein said at least one flow-through anode comprises at leastone of a carbon composite, a graphite rod, and a metal-graphite sinteredmaterial.
 18. The system according to claim 1, wherein said at least oneflow-through anode comprises a plurality of stainless steel rods. 19.The system according to claim 1, wherein said at least one flow-throughcathode is comprised of at least one of a plurality of parallel metalwires, a plurality of thin rods, a plurality of parallel metal strips, ametal mesh, an expanded porous metal structure, a metal wool and aconductive polymer.
 20. The system according to claim 19, wherein saidat least one flow-through cathode is polished.